Controls on Dissolved Silicon Isotopes Along the U.S. GEOTRACES Eastern Pacific Zonal Transect (GP16)

P Grasse,I Closset,M A Brzezinski,S Geilert,J L Jones

doi:10.1029/2020gb006538

Abstract

AbstractThe distribution of dissolved silicon isotopes (δ30Si) was examined along the U.S. GEOTRACES East Pacific Zonal Transect (GP16) extending from Peru to Tahiti (10°S and 15°S latitude). Surface waters in the subtropical gyre displayed high δ30Si due to strong utilization of silicic acid (DSi). In contrast, surface waters close to the Peruvian coast where upwelling prevailed were less depleted and only moderately fractionated. δ30Si of water masses along the transect was compared with the results of an Optimum Multiparameter Analysis that quantified the fractional contributions of end‐member water masses in each sample. Strong admixture of intermediate waters obscured the expected heavy isotopic signatures of Subantarctic Mode Water and Antarctic Intermediate Water. Isotope values were nearly homogenous below 2,000 m (average: +1.3 ± 0.1‰, 1 s.d.) despite the 25 μmol kg−1 range in the DSi content among water masses. This homogeneity confirms prior observations and model results that predict nearly constant δ30Si values of +1.0‰ to +1.2‰ for Pacific deep waters with [DSi] > 100 μmol kg−1. Waters above the East Pacific Rise (EPR) influenced by hydrothermal activity showed a small increase in [DSi] together with dissolved iron, but overall stations close to the EPR were slightly depleted in [DSi] (3 to 6 μmol kg−1) with no significant shift in δ30Si compared to adjacent waters. Hydrothermal [DSi] appears to precipitate within the conduit of the EPR or upon contact with cold seawater resulting in a negligible influence of hydrothermal fluids on δ30Si in deep water.

Highlights

Silicic acid, Si(OH)4, hereafter referred to as dissolved silicon (DSi), is one of the major macronutrients in the ocean along with nitrate (NO3−) and orthophosphate (PO43−)
Some interlaboratory comparisons have observed a constant offset between their δ30Si measurements, our observations confirm the previously published conclusion that there is no systematic offset between MC‐ICP‐MS and Isotope Ratio Mass Spectrometer (IRMS)
We present δ30Si data from the GEOTRACES GP16 transect from Peru to Tahiti. δ30Si was measured at two laboratories (GEOMAR and UCSB), and intercalibration between samples with high and low [DSi] resulted in excellent agreement

Summary

Introduction

Si(OH), hereafter referred to as dissolved silicon (DSi), is one of the major macronutrients in the ocean along with nitrate (NO3−) and orthophosphate (PO43−). Diatoms, which account for 40% of marine primary productivity, have a strict growth requirement for DSi that they use to build their cell wall DSi plays an essential role in oceanic primary productivity and is closely linked to the carbon cycle, especially in coastal upwelling areas, where diatoms dominate the phytoplankton community The isotopic composition of DSi (δ30Si) has proven to be a powerful tool to better understand the role of DSi and diatoms in ocean processes as the δ30Si of seawater carries information about DSi utilization in surface waters, the subsequent dissolution of sinking biogenic silica (hereafter referred to as BSi), as well as water mass mixing The isotopic composition of DSi (δ30Si) has proven to be a powerful tool to better understand the role of DSi and diatoms in ocean processes as the δ30Si of seawater carries information about DSi utilization in surface waters, the subsequent dissolution of sinking biogenic silica (hereafter referred to as BSi), as well as water mass mixing (e.g., De La Rocha et al, 1997; Grasse et al, 2013; B. Reynolds et al, 2006)

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